Exhaust gas guide section for an exhaust gas turbocharger and method for operating an exhaust gas turbocharger

ABSTRACT

The invention relates to an exhaust gas guide section for an exhaust gas turbocharger, with a first spiral channel (4) and a second spiral channel (5) for the inflow of a turbine wheel which is rotatably accommodated in the through-flow exhaust gas guide section, wherein a through-flow opening (10) is provided between the first spiral channel (4) and the second spiral channel (5), which is formed for inducing an overflow of exhaust gas from the first spiral channel (4) into the second spiral channel (5) and vice versa, and wherein the through-flow opening (10) comprises a movable covering element (9′) for opening or closing of the through-flow opening (10). According to the invention, the covering element (9′) is configured to degressively open a first flow cross section (11) of the through-flow opening (10).

TECHNICAL FIELD

The invention relates to an exhaust gas guide section for an exhaust gas turbocharger. Further, the invention relates to a method for operating an exhaust gas turbocharger.

BACKGROUND

Exhaust gas guide sections for exhaust gas turbochargers which comprise a control device for regulating a fluid flowing though the exhaust gas guide section, generally exhaust gas, are known. The control device is intended for opening and closing of a bypass duct in the through-flow exhaust gas guide section for bypassing a turbine wheel of the exhaust gas guide section, which is rotatable arranged in the exhaust gas guide section in a wheel chamber of the exhaust gas guide section. It comprises a cover element for opening and closing of a flow cross section of the bypass duct. By means of such a control device, it is possible to completely or partially bypass the turbine wheel in certain operating points of the exhaust gas turbocharger, in particular in operating points which exhibit large flow volumes so that an efficient operation of the exhaust gas turbocharger is made possible. The efficient operation of the exhaust gas turbocharger is dependent on a certain opening characteristic of the control device, which is to be adapted to the requirements of a combustion engine which is connected with the exhaust gas turbocharger.

From DE 198 24 913 A1, an exhaust gas guide section for an exhaust gas turbocharger with a control device may be taken, whose opening characteristic exhibits a progressive course. Depending on a travel of the cover element of the control device, an opening cross section of a bypass duct of the exhaust gas guide section progressively increases, in particular exponentially, with increasing travel starting from a closed position. Thus, a progressive opening characteristic is given which is suited for the described application.

WO 2010/039596 A2 also discloses an exhaust gas guide section for an exhaust gas turbocharger with a control device, whose cover element exhibits a progressive opening characteristic, starting from a closed position of the cover element.

Therefore, the object of the present invention is to provide an improved exhaust gas guide section for an exhaust gas turbocharger. It is another object to specify a method for operating an exhaust gas turbocharger.

SUMMARY

According to the invention, this object is solved by an exhaust gas guide section for an exhaust gas turbocharger with the features of claim 1. According to the invention, the other object is solved by a method for operating an exhaust gas turbocharger with the features of claim 13. Advantageous embodiments with suitable and non-trivial developments of the invention are indicated in the dependent claims.

According to the invention, an exhaust gas guide section for an exhaust gas turbo-charger comprises a first spiral channel and a second spiral channel for the inflow of a turbine wheel which is rotatably supported in the through-flow exhaust gas guide section. A through-flow opening is provided between the first spiral channel and the second spiral channel, which is formed for inducing an overflow of exhaust gas from the first spiral channel into the second spiral channel and vice versa. The through-flow opening comprises a movable covering element for opening or closing the through-flow opening. The covering element is configured to degressively open a first flow cross section of the through-flow opening. Is has proven to be of advantage that the first flow cross section is degressively opened for inducing an overflow from the first spiral channel into the second spiral channel, starting form a closed position of the cover element. The advantage of degressive opening is an improved controllability of the exhaust gas turbocharger, which is associated with improved response behaviour of the exhaust gas turbocharger. In addition, the degressive opening provides improved adjusting possibilities with further possible flow cross sections to be opened, e. g. a flow cross section of a bypass duct.

Another advantage of the possibility of an overflow from the first spiral channel into the second spiral channel by means of the covering element is the provision of the exhaust gas guide section which comprises the first spiral channel and the second spiral channel, each in the form of a usual partial spiral channel in such a manner that wrapping of the spiral channel about a longitudinal axis of the exhaust gas guide section covers only 180°, with a wrap formed around 360°. Thus, a smaller and more compact exhaust gas guide section may be formed.

In an embodiment of the inventive exhaust gas guide section, the exhaust gas guide section comprises a cover element for opening and closing a bypass duct of the exhaust gas guide section. The bypass duct is provided in the through-flow exhaust gas guide section for bypassing the turbine wheel and comprises a second flow cross section. The advantage is the possibility to not exclusively guide the exhaust gas from one spiral channel into the other and vice versa, but the exhaust gas of a mechanically and thermodynamically connected combustion engine may be guided under bypassing the turbine wheel, in particular in medium and high load and speed ranges. This leads to a further enhancement of the efficiency which may be achieved during operation of the exhaust gas turbocharger.

In another embodiment of the inventive exhaust gas guide section, the cover element is configured to degressively open the second flow cross section starting from a closed position into an intermediate position. As in the case of overflow of the turbine wheel, it was found that a degressive opening of the second flow cross section during the so-called flowing around or bypassing, respectively is advantageous for the application of the exhaust gas to the turbine wheel. The opening of the second flow cross section need not mandatorily be effected completely degressively, i. e. starting from the closed position into a second position, in which the second flow cross section is fully open. Thereby, the advantage of an improved controllability of the first flow cross section and of the second flow cross section is given.

A significant advantage of a combination of the degressive opening of the first flow cross section and of the second flow cross section is to be seen in that during operation with the combustion engine, a valve opening time of an outlet valve of the combustion engine may be advantageously extended. This may extend a load changing phase of the combustion engine for improved purging.

Preferably, the covering element or the cover element, respectively, may be moved by means of a control device. It is also conceivable to form a self-regulating covering element which automatically opens or closes by means of prevailing pressures in the spiral channels. However, a controllable regulation of the covering element by means of the control device is to be preferred, because this allows to actively influence the pressure ratio at the turbine wheel. Most preferably, the control device of the covering element corresponds to the control device of the cover element. This enables the movement of the covering element and of the cover element by means of a single control device, which again enables a compact construction of the exhaust gas turbocharger. A further advantage is a cost-efficient manufacture of the exhaust gas turbocharger due to a smaller number of components.

In a preferred embodiment, a movement of the cover element triggers a movement of the covering element. Therefore, the opening of the first flow cross section and of the second flow cross section can be directly influenced by a movement of the cover element. This means in other words that the movement of the covering element may be represented as a function of the cover element. This results in a simplified regulation of the two flow cross sections, e. g. in the case of a computer-controlled regulation of the flow cross sections.

For an even more compact construction of the exhaust gas turbocharger, the covering element corresponds to the cover element. In other words, this means that both the flow cross section and the second flow cross section may be opened and closed by means of one element only. In addition to a compact construction, this leads to a cost efficient manufacture of the exhaust gas turbocharger, because the number of components is reduced.

In order to achieve a degressive opening characteristic, the cover element and/or the covering element preferably comprises a pot-shaped outer contour. Such an outer contour provides in a simple manner a relatively large through-flow flow cross section already at the beginning of the opening, starting from the closed position of the cover element and/or of the covering element, which increases degressively over a certain travel of the cover element and/or of the covering element.

In another embodiment, the cover element and/or the covering element comprises a first element portion along its longitudinal axis and a second element portion, wherein a first angle of inclination of a first outer surface of the first element portion is greater than a second angle of inclination of a second outer surface of the second element portion for obtaining a degressive opening characteristic, wherein the second element portion is arranged opposite a second channel wall. The degressive opening characteristic is easily achievable by means of the two element portions. The cover element and/or the covering element may be manufactured in a simple process.

For sealing the first flow cross section or the second flow cross section, respectively, in the closed position of the cover element and/or of the covering element, it is provided with an annular surface which bears against a channel wall of the exhaust gas guide section. In addition, it is advantageous to place the ring of the covering element within the bypass duct. Thereby, the covering element is opened more easily by means of the amount of exhaust gas flowing though the bypass duct.

In another embodiment of the inventive exhaust gas guide section, the cover element and/or the covering element is mounted at a lever arm of the control device, which may be swivelled about an axis of rotation. Due to the possible rotational motion, relatively small actuation forces are required for moving the respective element.

In another embodiment of the inventive exhaust gas guide section, the covering element is configured to open the first flow cross section after the at least partially opened second flow cross section. This is advantageous in that overflow from the first spiral channel and the second spiral channel is possible only after the bypass duct has already opened. This makes it possible to avoid knocking of the combustion engine due to an excessive exhaust gas backpressure at the turbine wheel. At the same time, a potential reverse flow of overflowing exhaust gas into the spiral channel is prevented, whose thermodynamically connected cylinders are in an intake and/or combustion phase of a cyclic process of the combustion engine. In other words, whose outlet valve or outlet valves, respectively, are closed while the outlet valves or the outlet valve, respectively, of the cylinders which are connected with the other spiral channel are open.

In another embodiment, the cover element and/or the covering element is configured so that it may at least partially accommodate a wall of the exhaust gas guide section and/or vice versa. This ensures that in the closed position of the cover element there is no overflow from one spiral channel into the other one, or that in the closed position of the covering element, respectively, there is no flow-off into the bypass duct. Furthermore, it may be ensured, in particular when the covering element is formed like the cover element that e. g. the second flow cross section is opened before the first flow cross section, if this is required by a current operation of the combustion engine.

The second aspect of the invention relates to a method for operating an exhaust gas turbocharger with a through-flow exhaust gas guide section, wherein a bypass duct is formed for bypassing a turbine wheel which is rotatably supported in the exhaust gas guide section. The bypass duct comprises a second flow cross section which is to be opened and closed by means of a cover element. The second flow cross section is degressively opened, starting from a closed position of the cover element, in which the second flow cross section is completely closed.

By means of the inventive method, in particular in conjunction with the features of Sub-claims 14 to 17, a particularly efficient operation of the exhaust gas turbocharger combined with the combustion engine may be achieved. The inventive method allows the operation of the combustion engine with longer opening times of the outlet valve of the combustion engine. This may improve a charge change of the combustion engine due to a possible reduction of the exhaust gas backpressure in respect of a decrease of the so-called push-out work. Because longer opening times of the outlet valve of the combustion engine may be used at an enhanced efficiency of the combustion engine, a combustion engine exhibiting reduced consumption and thus emissions may be provided. In addition, the knocking tendency is considerably reduced.

Further advantages, features and details of the invention result from the following description of preferred exemplary embodiments as well from the drawing. The features and feature combinations as previously mentioned in the description as well as the features and feature combinations which will be mentioned in the following description of the figures and/or which are solely illustrated in the figures are not only applicable in the respective indicated combination but also in other combinations or isolated, without deviating from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows in a perspective view a section along a first sectional plane through an inventive exhaust gas guide section for an exhaust gas turbocharger.

FIG. 2 shows the exhaust gas guide section according to FIG. 1 with the control device in a second position.

FIG. 3 shows in a perspective view a section along a second sectional plane through the inventive exhaust gas guide section according to FIG. 1 with the control device in the first position.

FIG. 4 shows the exhaust gas guide section according to FIG. 3 with the control device in the second position.

FIG. 5 shows an s-A diagram of an opening characteristic of the inventive exhaust gas turbocharger.

FIG. 6 shows in a perspective view a cover element of the inventive exhaust gas guide section in a first variant.

FIG. 7 shows in a perspective view the cover element of the inventive exhaust gas guide section in a second variant.

FIG. 8 shows in a perspective view the cover element of the inventive exhaust gas guide section in a third variant.

FIG. 9 shows in a perspective view the cover element of the inventive exhaust gas guide section in a fourth variant.

FIG. 10 shows in a perspective view the cover element of the inventive exhaust gas guide section in a fifth variant.

FIG. 11 shows the cover element of the inventive exhaust gas guide section in a basic representation.

DETAILED DESCRIPTION

An inventive through-flow exhaust gas guide section 1 of an exhaust gas turbocharger 2 configured according to FIG. 1 comprises an inlet duct 3 for a fluid flow into the exhaust gas guide section 1, generally exhaust gas of a combustion engine 7, a first spiral channel 4 and a second spiral channel 5 downstream of the inlet duct 3 for conditioning of the flow and an outlet duct (not shown in detail) downstream of the spiral channels 4, 5, via which the exhaust gas may be discharged in a controlled manner from the exhaust gas guide section 1. Between the spiral channels 4, 5 and the outlet duct a wheel chamber (not shown in detail) is arranged in which a turbine wheel (not shown in detail) is rotatably supported.

The exhaust gas guide section 1 is connected with an exhaust gas manifold 6 of the combustion engine 7 so that the exhaust gas from the combustion engine 7 may enter the spiral channels 4, 5 via the inlet duct 3 to be applied to the turbine wheel.

For adapting the operational behavior of the exhaust gas turbocharger 2 to the fluid flow of the combustion engine, and thus to the combustion engine, a control device 8 for disconnecting and connecting the first spiral channel 4 and the second spiral channel 5 is arranged in the exhaust gas guide section 1. For disconnecting and for connecting the two spiral channels 4, 5 a covering element 9′ of the control device 8 is arranged in a through-flow opening 10 which is formed to provide a through-flow through the two spiral channels 4, 5.

In a first position, the closed position of the covering element 9′ as shown in FIG. 1, a through-flow through the two spiral channels 4, 5 is effected completely separately from one another, with the through-flow opening 10 being completely closed by means of the covering element 9′. The exhaust gas from the combustion engine flows through the two spiral channels 4, 5, with a first portion of the exhaust gas flowing through the first spiral channel 4 and a second portion of the exhaust gas flowing through the second spiral channel 5.

In a second position of the covering element 9′ as shown in FIG. 2, the through-flow opening 10 is completely opened and exhaust gas may overflow from the first spiral channel 4 into the second spiral channel 5 and vice versa. This means that exhaust gas may overflow from the one spiral channel 4; 5 into the other spiral channel 5; 4 via the through-flow opening 10 which has a first flow cross section 11.

The covering element 9′ is to be positioned in further intermediate positions between the first position and the second position so that the first flow cross section 11 may be adapted to a corresponding requirement for achieving an optimum efficiency of the exhaust gas turbocharger in relation to the quantity of the exhaust gas flowing through. It was found that a degressive opening of the first flow cross section 11 starting from the first position, i. e. from the completely closed first flow cross section 11, towards the second position, and thus towards a completely opened first flow cross section 11, is advantageous. In other words, the covering element 9′ is formed for inducing a degressive opening of the first flow cross section 11. This means that the control device 8 exhibits a degressive opening characteristic for opening of the first flow cross section 11.

For inducing the degressive opening of the first flow cross section 11, the covering element 9′ comprises a pot-shaped outer contour 12. It comprises a base plate 13 which accommodates an element body 15 at a first plate surface 14. An actuating device 17 of the control device 8 is fixed at the base plate 13 on a second plate surface 16 opposite the element body 15.

The element body 15 has a body basic cross section KQ which is smaller than a plate cross section PQ of the first plate surface 14. Thereby, an annular surface 18 is formed which completely surrounds the element body 15. This annular surface 18 serves to support the covering element 9′ against a first channel wall 19 of the through-flow opening 10, on the one hand, and, on the other hand, as a sealing surface for sealing the two spiral channels 4, 5 in the first position of the covering element 9′.

For achieving the degressive opening characteristic, the element body 15 comprises two element portions, a first element portion 20 and a second element portion 21, which are arranged in series in the direction of a longitudinal axis 22 of the element body 15. The first element portion 20 is firmly connected with the base plate 13 and is arranged between the second element portion 21 and the base plate 13. The second element portion 21 is also firmly connected with the first element portion 20. The second element portion 21 comprises a top surface 23 of the covering element 9′, which faces away from the first element portion 20.

FIG. 11 shows the covering element 9′ in a basic representation. The first element portion 20 and the second element portion 21 comprise essentially inclined outer surfaces 24, 25 from the base plate 13 towards the top surface 23 in the direction of the longitudinal axis 22. A first angle of inclination α1 of the first outer surface 24 of the first element portion 20 is greater than a second angle of inclination α2 of the second outer surface 25 of the second element portion 21.

In a first variant of the cover element 9 or of the covering element 9′, respectively, according to FIGS. 1 to 4, 6, both the body basic cross section KQ and the plate cross section PQ have an oval shape. The element body 15 comprises a groove 28 with a width BR, which is formed extending across the top surface 23 over its circumference along a transverse axis 26. In order to reliably seal the first spiral channel 4 and the second spiral channel 5 in the first position, a second channel wall 29 opposite the first channel wall 19 comprises a projection 30 which is formed complementary to the groove.

In a second variant according to FIG. 7, the top surface 23 of the covering element 9′ has a plane configuration. In a third and a fourth variant according to FIGS. 8 and 9, the covering element 9′ comprises a circular cross section. In the third variant, the second element portion 21 is cone-shaped, while it is configured in the form of spherical layers in the fourth variant.

The fifth variant of the covering element 9′ shown in FIG. 10 also comprises a pot-shaped outer contour 12, with the top surface 23 being formed rectangular, so that the outer contour 12 of the second element portion 21 has an increased slope in the area of the top surface 23, which differs from the remaining area of the second element portion 21.

In addition, a bypass duct 31 is provided in the through-flow exhaust gas guide section 1 for bypassing the wheel chamber in the exhaust gas guide section 1. The bypass duct 31 comprises an inlet opening 32 with a second flow cross section 33, wherein the inlet opening 32 may be completely or partially closed or completely or partially opened, respectively, by means of the control device 8 or the cover element 9, respectively.

The cover element 9 of the control device 8, which corresponds to the covering element 9′, is rotatable accommodated in the bypass duct 31, and thus in the exhaust gas guide section 1, with the cover element 9 being supported for swivelling about an axis of rotation 34 of the cover element 9 in the bypass duct 31 and thus in the exhaust gas guide section 1. This means in other words, the cover element 9 is swivel-mounted in the exhaust gas guide section 1, wherein the second flow cross section 33 of the inlet opening 32 may be adjusted by means of the swivelling motion.

By means of the cover element 9, a predominantly degressive opening of the second flow cross section 33 may be achieved over a wide opening range, see FIG. 4. Therefore, only a small exhaust gas quantity may be discharged via the second flow cross section 33 into the bypass duct 31 until the first flow cross section 11 is completely open. With the first flow cross section 11 completely open, the second flow cross section 33 is also completely open, so that overflow from the first spiral channel into the second spiral channel 5 and vice versa and simultaneously blow-off of exhaust gas past the turbine wheel may take place.

To provide movability of the cover element 9 and thus of the control device 8, a cylinder-type opening 35 is formed in the exhaust gas guide section 1, in which a shaft 36 with a second longitudinal axis 37 of the control device 8 is rotatably accommodated. A lever arm 38 between the shaft 36 and the cover element 9 serves to connect the cover element 9 and the shaft 36 so that the cover element 9 may be swivelled by a rotation of the shaft 36.

FIG. 5 shows an s-A diagram, wherein s denotes a standardized travel of the cover element 9 or of the covering element 9′, respectively, and AF identifies a free flow cross section of the flow cross sections 11, 33 depending on the travel s. The travel s is a function of an opening angle of the cover element 9 or of the covering element 9′, respectively, because the covering element 9′ corresponds to the cover element 9. The opening angle with a value 0° corresponds to the closed position of the cover element 9 or of the covering element 9′, respectively.

A first line A represents the first flow cross section 11 over the travel s of the covering element 9′ according to the exemplary embodiment. A second line B represents the free second flow cross section 33 which is variable by means of the cover element 9. In a variant, a third line C represents the free first flow cross section 11. The courses of lines A, B, C describe an opening characteristic of the control device 8.

According to the first line A as well as to the third line C, the control device 8 exhibits a degressive opening characteristic of the first flow cross section 11. This means in other words that the first flow cross section 11, starting from a closed position of the covering element 9′, in which the first flow cross section 11 is completely closed, is degressively opened.

According to the course of line B, the control device 8 exhibits a degressive opening characteristic in relation to the second flow cross section 33. This means in other words that the second flow cross section 33, too, starting from a closed position of the cover element 9, in which the second flow cross section 33 is completely closed, is degressively opened.

In the variant according to line C, the first flow cross section 11 is only opened when the second flow cross section 33 has already been partially opened. Preferably, opening of the first flow cross section 11 takes place at an opening angle of approx. 5° of the cover element 9. Most preferable, the opening angle ranges from 0° to 3°.

In order to prevent overflow of exhaust gas from the first spiral channel 4 into the second spiral channel 5 and vice versa upon opening of the first flow cross section 11 according to line A, the first flow cross section 11 is smaller than the second flow cross section 33 up to a travel s1 of the cover element 9. If the first flow cross section 11 is opened according to line C, the first flow cross section 11 has to be made smaller than the second flow cross section 33 up to a travel s1′. 

1. An exhaust gas guide section for an exhaust gas turbocharger, with a first spiral channel (4) and a second spiral channel (5) for the inflow of a turbine wheel which is rotatably accommodated in a through-flow exhaust gas guide section, wherein a through-flow opening (10) is provided between the first spiral channel (4) and the second spiral channel (5), which is formed for inducing an overflow of exhaust gas from the first spiral channel (4) into the second spiral channel (5) and vice versa, and wherein the through-flow opening (10) comprises a movable covering element (9′) for opening or closing of the through-flow opening (10), and wherein the covering element (9′) is configured to degressively open a first flow cross section (11) of the through-flow opening (10).
 2. The exhaust gas guide section according to claim 1, wherein the exhaust gas guide section comprises a cover element (9) for opening and closing a bypass duct (31) of the exhaust gas guide section (1), and wherein the bypass duct (31) is provided in the through-flow exhaust gas guide section (1) for bypassing the turbine wheel, and wherein the bypass duct (31) comprises a second flow cross section (33).
 3. The exhaust gas guide section according to claim 2, wherein the second flow cross section (33) may be degressively opened by means of the cover element (9) at least starting from a closed position into an intermediate position.
 4. The exhaust gas guide section according to claim 2, wherein a control device (8) is provided in the exhaust gas guide section (1) for moving the covering element (9′) and/or the cover element (9).
 5. The exhaust gas guide section according to claim 2, wherein a movement of the covering element (9′) triggers a movement of the cover element (9).
 6. The exhaust gas guide section according to claim 2, wherein the covering element (9′) corresponds to the cover element (9).
 7. The exhaust gas guide section according to claim 2, wherein the cover element (9) and/or the covering element (9′) comprises a pot-shaped outer contour (12).
 8. The exhaust gas guide section according to claim 2, wherein the cover element (9) and/or the covering element (9′) comprises a first element portion (20) and a second element portion (21) along its longitudinal axis (22), wherein a first angle of inclination (α1) of a first outer surface 24 of the first element portion (20) is greater than a second angle of inclination (α2) of a second outer surface (25) of the second element portion (21) for inducing a degressive opening characteristic, wherein the second element portion (21) is arranged opposite a second channel wall (29).
 9. The exhaust gas guide section according to claim 2, wherein the cover element (9) and/or the covering element (9′) comprises an annular surface (18), which bears against a channel wall (19) of the exhaust gas guide section (1).
 10. The exhaust gas guide section according to claim 2, wherein the cover element (9) and/or the covering element (9′) is mounted at a lever arm (38) of the control device (8, 8′), which may be swivelled about an axis of rotation (34).
 11. The exhaust gas guide section according to claim 2, wherein the covering element (9′) is configured to open the through-flow opening (10) after the at least partially opened second flow cross section (33).
 12. The exhaust gas guide section according to claim 11, wherein the cover element (9) and/or the covering element (9′) is formed for accommodating a wall of the exhaust gas guide section (1) at least partially and/or vice versa.
 13. A method for operating an exhaust gas turbocharger, with a through-flow exhaust gas guide section (1) comprising a first spiral channel (4) and a second spiral channel (5) for the inflow of a turbine wheel which is rotatably accommodated in the exhaust gas guide section (1), wherein a through-flow opening (10) is provided between the first spiral channel (4) and the second spiral channel (5), which is formed for inducing an overflow of exhaust gas from the first spiral channel (4) into the second spiral channel (5) and vice versa, and wherein the through-flow opening (10) comprises a movable covering element (9′) for opening or closing of the through-flow opening (10), characterised in that a first flow cross section (11) of the through-flow opening (10) is degressively opened starting from a closed position of the covering element (9′).
 14. The method according to claim 13, wherein a bypass duct (31) for bypassing the turbine wheel is formed in the exhaust gas guide section (1), and wherein the bypass duct (31) comprises a second flow cross section (33), and wherein the second flow cross section (33) is opened by means of a cover element (9), starting from a closed position of the cover element (9).
 15. The method according to claim 14, wherein the second flow cross section (33) is degressively opened.
 16. The method according to claim 14, wherein the first flow cross section (11) is smaller than the second flow cross section (33) of the bypass duct (31) up to a travel (s1, s1′) of the covering element (9′).
 17. The method according to claim 14, wherein the first flow cross section (11) is opened after the second flow cross section (33). 